A hybrid fiber coaxial (HFC) technology is an integrated digital service broadband network access technology. An access network using the HFC technology generally includes three parts: a fiber trunk, a coaxial cable branch, and a user distribution cable. A program signal from a cable television broadcast station is first converted into an optical signal for transmission on the fiber trunk. The optical signal is converted into an electrical signal after arriving in a user area. After being distributed by using a distributor, the electrical signal is sent to a user by using the coaxial cable branch and the user distribution cable.
As shown in FIG. 1, according to a recording of a specification CM-SP-OSSI disclosed by a Cable Television standard organization, Cable Labs, a network that complies with a Data Over Cable Service Interface Specification (DOCSIS) falls into the following types: a backend office network, an HFC network, and a home network. The backend office network is connected to the HFC network by using a cable modem termination system (CMTS), and the backend office network includes a network management system (NMS) and a provisioning server. On a central office side, the HFC network is connected to the backend office network by using the CMTS; on a user side, the HFC is connected to customer-premises equipment (CPE) by using a cable modem (CM), where the CPE may fall into the following types according to a supported Internet Protocol version: CPE supporting an IPv4 and CPE supporting an IPv6.
A typical HFC network generally includes various devices (such as a CMTS, a CM, and an optical node), components (such as an amplifier, an attenuator, a splitter, and a distributor), and lines (such as an optical cable and a cable). A fault may occur on each device, or each component, or each segment of the cable or the optical cable.
During network operation maintenance (Network Maintenance), an operating situation of a network can be known by monitoring a signal (such as parameters of a network and a device) on the network, and when a fault occurs on the network, fault locating and fault clearing are performed. Conventional network maintenance is triggered by a complaint from the user and is passive. With development of technologies, a proactive network maintenance (PNM) technology has developed. When the PNM technology is applied to the HFC network, by analyzing parameters of the HFC network and a device, an operating situation of the HFC network is known in real time, a problem is found and located in advance, and a fault is handled before the user makes a complaint.
Generally, a location of a PNM server on the HFC network is shown in FIG. 2. The PNM server collects operating parameters of the CMTS and the CM from a local area network or a wide area network, and then performs network maintenance according to the operating parameters.
A solution of performing HFC network maintenance according to the operating parameters is as follows: manually analyzing a constellation diagram of signals received over a communications link on the HFC network (a test instrument is generally connected to a particular part of the HFC network to measure and display the constellation diagram), and monitoring a fault (for example, at a particular point of the communications link on the HFC network, a signal is received by using the test instrument, and the received signal is measured to display the constellation diagram). The following briefly describes principles of performing HFC network maintenance by manually analyzing the constellation diagram to know the operating situation of the network and monitor the fault.
In-phase quadrature (IQ) modulation is also referred to as quadrature amplitude modulation (QAM). In a digital communications system, two orthogonal signals are generally used for modulation, and a real part and an imaginary part of a signal are respectively correspondingly modulated to two orthogonal signals I and Q. A modulated or demodulated digital signal can be indicated by using a complex number (I, Q) constellation point. If the complex number is drawn onto a complex plane, the complex number is corresponding to a point on the complex plane, and therefore the complex number becomes a constellation point. By drawing multiple constellation points onto the complex plane, a constellation diagram is formed. Both a PNM system and a conventional test instrument can provide function maintenance for the constellation diagram.
The constellation diagram has different features. There are N quadrate grids in the constellation diagram, and N is corresponding to a QAM order; for example, for 64-QAM, there are 64 grids on the complex plane. In a normal constellation diagram, constellation points are all distributed in the centers of grids, as shown in FIG. 3.
Different faulty constellation diagrams have different features. The following are some common faulty constellation diagrams.
1. When a phase error occurs, constellation points rotate (as shown in FIG. 4).
2. When a gain compression error occurs, peripheral constellation points deviate towards the center (as shown in FIG. 5).
In addition, other faults also have different features; for example, a constellation diagram is in a rectangular shape, in a rhombic shape, or the like.
The foregoing solution of manually analyzing a received-signal constellation diagram to know an operating situation of a network and detect a network fault has the following disadvantages:
1. A constellation diagram observation method is applicable only to a modulation scheme with a small QAM order. It is generally considered that the constellation diagram cannot be used when a QAM order is greater than 4096, because neither observation nor analysis can be performed when many grids are displayed in one diagram. A conventional HFC network does not use high-order modulation (256 QAM to a maximum extent, that is, there are a maximum of 256 grids in the constellation diagram), and therefore a method for manually analyzing a constellation diagram for fault determining is still applicable. However, on a next-generation HFC network, a modulation order is greatly increased (set to up to 16384 QAM), and such a high order is far beyond an applicable scope of manual analysis of the constellation diagram. Currently, there is not an effective method that can be used to perform fault analysis on a constellation diagram with such a high QAM order.
Currently, the DOCSIS supports 16384 QAM to a maximum extent, and such a high order is far beyond an applicable scope of the constellation diagram.
2. The constellation diagram needs to be analyzed by an engineer having some technical knowledge. For manual analysis of the constellation diagram, there is a problem that an analysis result depends on the engineer's technical level and the result may be inaccurate.
In conclusion, the solution of performing network maintenance by manually analyzing the received-signal constellation diagram is not applicable to a higher-order QAM scheme, and an analysis result depends on an technical level.